Heterogeneous catalysis processes, using metal catalysts, are of commercial importance in a large number of chemical and petrochemical processes. Examples include hydrocarbon cracking and reforming, dehydrogenation reactions, isomerization reactions, Fischer-Tropsch processes and many others. The economic performance of such processes depends to a large extent on the activity of the catalyst, the selectivity of the catalyst towards the desired reaction product, and the cost and complexity of preparation of the catalyst in its most advantageous form for use in the particular process under consideration.
Whilst there are still scientific uncertainties about the precise nature of heterogeneous catalysis reactions, it is widely accepted that such catalysis is largely a surface phenomenon, so that most efficient catalysts should normally have large metallic surface area, achieved by producing the catalyst in very small particle form.
Recently it has been recognized that metals exhibit very high heterogeneous catalytic activity when they are present in monatomic or small cluster form, i.e. from 1 to about 100 metal atoms per cluster. They are normally zero-valent, but may also be in various oxidation states. Such clusters may be unimetallic or bimetallic. The properties of such clusters are quite different from those exhibited by bulk, colloidal metal deposits, at least as regard catalytic activity and selectivity.
Such metal clusters are, however, extremely short lived and unstable under normal conditions. When bulk metals, especially transition metals, are vaporized eg. by resistive heating or electron bombardment, the initially formed vapor is in the monatomic condition. Very rapidly indeed, under normal conditions, the single metal atoms agglomerate into small clusters on a surface, and then very rapidly form bulk, colloidal metal by further agglomeration.
There is accordingly a need for relatively simple and efficient, economically attractive methods for producing supported metal catalysts having the metal deposited thereon in monatomic or small cluster form, and in which the metal deposits are stable in such form.
As used herein, the term "supported metal cluster" refers to metal atoms bonded to a support and weakly or strongly bonded to one another, a significant proportion of the metal atoms being in a reduced or zero-valent state, each supported cluster comprising up to about 100 metal atoms. Such a supported cluster includes any aggregation of two or more metal atoms of the same or different species, regardless of whether they occur in substantially one dimensional form (i.e. a chain of metal atoms), or two dimensional forms (i.e. a planar arrangement), or a three dimensional structure.